This application is based upon and claims the benefit of priority of Japanese Patent Applications No. 2000-379056 filed on Dec. 13, 2000, No. 2001-174102 filed on Jun. 8, 2001 and No. 2001-263496 filed on Aug. 31, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a yoke of an electric rotating machine having a magnet installation portion whose wall thickness is locally thicker and a method of manufacturing the same.
2. Description of Related Art
Generally, a magnet installation portion of a motor yoke has a relatively thick wall thickness so that a better magnetic characteristic of motor is secured. If the yoke is formed by a conventional manufacturing method in use of a plate whose thickness is equal to the wall thickness of the magnet installation portion necessary for securing a given magnetic characteristic, wall thickness of a portion of the yoke other than the magnet installation portion tends to become too thick so that a lighter weight motor can not be manufactured since a total weight of the yoke is heavier.
According to another conventional method, a separately provided ring is fitted to an outer cylindrical wall portion of the yoke corresponding to the magnet installation portion, as shown in FIG. 15. In more details, a cylindrical portion 41 of a yoke 40 is made, at first, of a relatively thin thickness plate (original plate) in a given shape. Then, an iron ring 43, whose inner diameter is equal to an outer diameter of the cylindrical portion 41 and whose axial length is substantially equal to an axial length of a magnet installation portion 42 of the cylindrical portion 41, is separately provided and is fitted to the cylindrical portion 41 so as to surround an outer circumference 40a thereof. Thickness of the iron ring 43 is equal to or more than the thickness of the cylindrical portion 41 so that a final wall thickness of the magnet installation portion 42 becomes thicker by twice or more than that of the original plate, resulting in improving the magnetic characteristic of the yoke.
However, the method shown in FIG. 15 has a drawback that the yoke 40 is made of two parts and the two parts have to be assembled so that manufacturing cost of the yoke 40 increases. Further, unless the two parts are manufactured with accurate dimensions, a gap between the two parts is produced. The gap is likely to cause corrosion since an inside of the gap is hardly coated with paint.
Further, it is known to make the thickness of the magnet installation portion partially thicker in such a manner that, while the cylindrical portion of the yoke is formed by drawing, the magnet installation portion is axially compressed through a compression forming process. However, this method still has a drawback that the thickness of the magnet installation potion increases only by 1.3 to 1.6 times than the thickness of the original plate because the thickness increase due to the compression forming process is inherently limited. Accordingly, to make the thickness of the magnet portion largely increase, it is necessary to repeat the compression forming process many times, resulting in increasing a number of the yoke forming processes.
An object of the invention is to provide a method of forming easily a cup shaped yoke of an electric rotating machine out of a piece of plate at a lower manufacturing cost so that wall thickness of the magnet installation portion is substantially equal to thickness of the plate and, further, wall thickness of the bottom portion, or, preferably, wall thickness of a portion other than the magnet installation portion, is thinner than the thickness of the plate.
Another object of the invention is to provide an integrally formed cup shaped yoke of an electric rotating machine whose side wall has an opening portion, a magnet installation portion and a shoulder portion and whose bottom wall has a bottom portion with a boss for accommodating a bearing in which wall thickness of the magnet installation portion is thicker than thickness of the bottom wall, or, preferably, each thickness of the opening and the shoulder portion.
To achieve the above objects, in a method of manufacturing a cup shaped yoke of an electric rotating machine, whose side wall has an opening portion, a magnet installation portion and a shoulder portion and whose bottom wall has a bottom portion with a boss, out of a piece of plate, the plate is locally bulged to form a first worked plate having a semispherical protrusion by pressing a first area of the plate, after firmly holding the plate around an outside of the first area with a first holding force so that wall thickness of the first semispherical protrusion is thinner than a thickness of the plate.
Then, the first worked plate is drawn to form a second worked plate having a first cup by pressing a second area of the first worked plate at and around an outside of the semispherical protrusion, whose diameter is larger than that of the first area, after holding the first worked plate around an outside of the second area with a second holding force that is lower than the first holding force so that thickness of a bottom wall of the first cup is thinner than the wall thickness of the semispherical protrusion and a thickness of a side wall of the first cup is substantially equal to the thickness of the plate.
After that, the bottom wall of the first cup is bulged to form the boss in a center thereof.
According to the method mentioned above, the cup shaped yoke is easily manufactured so that the thickness of the side wall is substantially equal to the thickness of the original plate and thicker than the thickness of the bottom wall.
Alternatively, the cup shaped yoke may be manufactured as follows.
At first, the plate is drawn to form a first worked plate having a first cup by pressing a second area of the plate, after holding the plate around an outside of the second area with a second holding force so that each wall thickness of a side wall and a bottom wall of the first cup is substantially equal to thickness of the plate.
Then, the first worked plate is bulged to form a second worked plate whose bottom wall has a semispherical protrusion by pressing a first area of the bottom wall of the first cup, whose diameter is smaller than that of the second area, after firmly holding the first worked plate around an outside of the first area with first holding force that is higher than the second holding force so that wall thickness of the semispherical protrusion of the second worked plate is thinner than that of the bottom wall of the first cup of the first worked plate and wall thickness of a side wall of the second worked plate is substantially equal to that of the side wall of the first cup of the first worked plate.
After that, the bottom wall including the semispherical protrusion of the second worked plate is bulged to form the boss in a center thereof.
This method will also effective to easily manufacture the cup shaped yoke.
It is preferable in both methods mentioned above that the diameter of the second area is larger than an inside diameter of the cup shaped yoke. In this case, as a next step after forming the second worked plate, while holding a third area of the bottom wall of the first cup of the second worked plate, whose diameter is smaller than that of the second area, the second worked plate is further drawn to form a third worked plate having a second cup whose inside diameter is smaller than that of the first cup and whose axial length is longer than that of the first cup by pressing the second worked plate at an outer periphery of the bottom wall of the first cup axially along an outer circumference of the third area. Accordingly, the outer periphery of the bottom wall of the second worked plate is moved to constitute a part of a side wall of the third worked plate.
Furthermore, it is preferable that the third worked plate is further deformed to form a fourth worked plate by ironing opposite axial circumferential edges of the side wall of the second cup so that the fourth worked plate has the magnet installation portion, the opening portion and the shoulder portion in which each wall thickness of the opening and shoulder portions is thinner than the wall thickness of the magnet installation portion. Preferably, each wall thickness of the opening and shoulder portions is not higher than a half of the wall thickness of the magnet installation portion.
It is preferable that an outer circumference of the shoulder portion and an inner circumference of the opening portion are ironed.